6 min read Essential Bit Operations x & y # AND: both bits must be 1 x | y # OR: at least […] Read article
6 min read Requirements Detect fraudulent transactions in real time (<200ms per transaction decision) Flag suspicious user behavior: account takeover, card testing, synthetic […] Read article
5 min read Core Idea Build a prefix array where prefix[i] = arr[0] + arr[1] + ... + arr[i-1]. Then any range sum […] Read article
7 min read Two Graph Models The first design decision is the relationship model: Directed graph (Twitter/Instagram follow): A follows B does not […] Read article
6 min read An e-commerce platform touches nearly every backend concept that appears in system design interviews: relational data modeling, inventory consistency, distributed […] Read article
7 min read Stock trading dynamic programming problems are a classic interview cluster. Every variant – one transaction, unlimited, at most k, cooldown, […] Read article
9 min read A ticket booking system like Ticketmaster must handle concurrent seat selection, hold expiry, payment confirmation, and flash sale traffic spikes. […] Read article
9 min read Advanced Binary Search Interview Patterns Binary search is not just “is target in sorted array.” The real interview value comes […] Read article
10 min read Recommendation System Low-Level Design Recommendation systems are one of the highest-ROI components in consumer products. They solve the discovery problem: […] Read article
7 min read Requirements Functional requirements for a URL shortener: Create a short URL from a long original URL Redirect users from the […] Read article